The present invention relates to a system for maintaining an accurate frequency source in a single sideband AM radio repeater station.
In single sideband AM radio, since no carrier is present, frequency errors at IF and RF appear as direct errors at the voice frequency. Therefore, maintaining an accurate reference frequency is essential.
An approach to achieving frequency accuracy throughout the system is to improve the long term stability of all translation carriers by means of external reference. Another approach uses techniques by which long term stability of the transmit and receive ends can be coupled to each other by the transmission of appropriate pilots. The present invention represents a part of a system in which overall transmission error is reduced to a small, but non-zero amount, enabling the IF demodulator at the receive terminal to be phase locked to a voltage-controlled crystal oscillator, resulting in essentially error free baseband.
One system for establishing the receive and transmitter frequencies in a repeater station begins with generating a microwave signal at 6 GHz. A sample of the microwave signal is mixed with a 252 MHz signal to produce a receive frequency 252 MHz away from the transmit frequency. This type of system has disadvantages associated with it. Since the frequency generation is done at microwave frequency, and communication is by means of waveguide, it is important that the transmitter and receiver be colocated. Further, there are occasions when the standard frequency differential between transmitter and receiver is not available, for example, because one of the frequencies has been preempted. Changing the frequencies of operation in this type of system is an expensive undertaking.